This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. ACERT facilities presently include a 170/240GHz quasioptical-bridge spectrometer for c.w. EPR studies. During the past year, the spectrometer bridge has been substantially upgraded, e.g., with replacement of the c.w. millimeter-wave sources and installation of a newly-designed reference arm. Additionally, the spectrometer resonator has benefited from a series of mechanical improvements which have significantly improved vibration isolation and tuning accuracy. Our goal in pursuing the fabrication and implementation of these items is to create an instrument of exceptional capability while maintaining relative ease of use for less-experienced operators. With that objective in mind, we intend to continue our development of an automatic frequency control (AFC) system for the 170/240GHz c.w. spectrometer suitable for use with either of the recently-acquired millimeter-wave sources. In common with familiar AFC systems found in conventional c.w. microwave EPR instruments, our proposed high-field AFC systems will provide conventional source frequency-tracking capability, thereby eliminating detrimental effects of resonator or source frequency drift during the course of an experiment. In our conceptual design, it will also be possible to configure the AFC system in a way which simplifies adjustment of the Fabry-Perot resonator by allowing adjustment of sample position without the potential for operator confusion due to detuning effects caused by corresponding variation in the sample's dielectric loading. Preliminary design measurements and studies of the AFC loop have been made and a conceptual design completed. We have tentatively scheduled construction, test and final implementation of this spectrometer upgrade for later this year.